Change equation formatting to code formatting

build_finetune_functions is being rendered as a broken math equation, not a code snippet.

Author: zacstewart

doc/SdA.txt

@@ -410,7 +410,7 @@ to the training set for a fixed number of epochs given by
 
 The fine-tuning loop is very similar with the one in the :ref:`mlp`, the
 only difference is that we will use now the functions given by
-`build_finetune_functions`  .
+``build_finetune_functions``  .